In an Orthogonal Frequency Division Multiplexing (OFDM) communication system, a frequency bandwidth is split into multiple contiguous frequency sub-bands, or sub-carriers, that are transmitted simultaneously. In an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, a user may then be assigned one or more of the frequency sub-bands for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different sub-carriers. These sub-carriers are orthogonal to each other, and thus there is no inter-sub-carrier interference in theory This invention can be applied to both OFDM systems, where all sub-carriers are assigned to a single user, and OFDMA systems, where the sub-carriers are shared by multiple users in the same time. In what follows, the OFDM and OFDMA are interchangeable.
In an OFDMA receiver, mixers translate a high input radio frequency (RF) to a lower intermediate frequency (IF). This process is know as downconversion and it utilizes a difference term between the mixer's RF input and a local oscillator (LO) input for low side injection (LO frequency<RF frequency), or a difference term between the mixer's LO and RF inputs for high-side injection. This downconversion process can be described by the following equation,fIF=±fRF±fLO,where fIF is the intermediate frequency (IF) at the mixer's output port, fRf is any RF signal applied to the mixer's RF input port, and fLO is the local oscillator signal applied to the mixer's LO input port.
Ideally, an amplitude and a phase of the mixer output signal are proportional to the RF input signal's amplitude and phase and are independent of the LO signal characteristics. However, in practice, mixer nonlinearities produce undesired mixing products, called spurious responses, or spurs, which are caused by undesired signals reaching the mixer's RF input port and producing a response at the IF frequency. The signals reaching the RF input port do not necessarily have to fall into the desired RF band to be troublesome. Many of these signals are sufficiently high in power level that the RF filters preceding the mixer don't provide enough sensitivity to keep them from causing additional spurious responses. When they interfere with the desired IF frequency, the mixing mechanism can be described by the following equation,fIF=±mfRF±nfLO,where m and n are integer harmonics of both the RF and LO frequencies that mix to create numerous combinations of spurious products.
In reality, the amplitude of these spurious components decreases as the value of m or n increases. By knowing the desired RF frequency range, frequency planning may be used to carefully select the IF and the corresponding LO frequencies to avoid spurious mixing products whenever possible. Filters are then used to reject out-of-band RF signals that might cause in-band IF responses. Intermediate frequency (IF) filter sensitivity following the mixer is then specified to pass only the desired frequencies, thereby filtering the spurious response signals ahead of the final detector. However, spurious responses that appear within the IF band will not be attenuated by the IF filter. This technique of spur avoidance works well in case of narrow band RF signals.
For wireless broadband systems, such as 802.16e, RF signal bandwidth could be more than 20 MHz and it is very difficult to avoid spurious responses in the desired IF bandwidth. For example, suppose a 768 MHz sinusoidal clock signal is fed to transmitter digital-to-analog devices (DAC) in an 802.16e Base Station (BS), where the clock signal is divided by two to produce a square wave clock signal at 384 MHz. If the receiver is tuned to exactly 3.456 GHz, the ninth harmonic of this signal is also a sinusoid at 3456 MHz. This continuous waveform may be radiated from a digital section of a transceiver (TRX) board inside a casting of a transceiver and may be coupled into the front-end circuitry of the receiver side of the transceiver, where the signal gets mixed down to the IF within the desired signal bandwidth. Mathematically, this can be expressed as[(768 MHz/2)×9]−(LO frequency 2988 MHz)=3.456 GHz−2988 MHz=IF frequency 456 MHzwhere the LO frequency is 2988 MHz to get an IF frequency of 456 MHz.
Unlike other wireless broadband systems such as CDMA, where processing gain naturally provides spur suppression, an OFDMA system is very susceptible to the spur inside the desired signal bandwidth. It is very difficult to avoid spurious responses for wireless broadband OFDMA signals in RF design. Therefore a need exists for a simple and effective method of digital spur cancellation that may be implemented in baseband processing.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.